Method of manufacturing double layer attenuation panel with two layers of linear type material

ABSTRACT

A method of manufacturing a double degree acoustic attenuation sandwich panel having a plurality of stacked components adhered together to form a unitary sandwich structure. The structure comprising an impervious facing of thin sheet material, a first honeycomb core with end wise directed cells, a first perforate facing of thin sheet material, a first thin layer of porous fibrous material, a second perforated facing of thin sheet material, a second honeycomb core with end wise directed cells, a third perforate facing of thin sheet material, the second perforated facing sheet having substantially larger perforations than the first and third sheets, and a second layer of porous fibrous material. The resulting sandwich panel has a pre-determined flow through resistance between the outer surface of the first and second layers of porous fibrous material and the cells of the first and second honeycomb cores. The cells of the first and second honeycomb cores may be of equal or different volume and may be constructed of similar or dissimilar materials.

BACKGROUND OF THE INVENTION

This invention relates to a method of manufacturing an improved noiseattenuation structure, more particularly to a method of manufacturing astructure having a first impervious facing sheet adhered to one surfaceof a first honeycomb core, a first perforate facing sheet with a firstlayer of porous fibrous material adhered to the other surface of thefirst honeycomb core, a second perforated facing sheet adhered to theporous fibrous woven material, the opposite surface of the secondperforated sheet adhered to one surface of a second honeycomb core andthe other surface of the second honeycomb core adhered to a thirdperforated facing sheet with a second layer of porous fibrous materialsecured to the outside or opposite surface of the third perforatedsheet. A continuous communication at a predetermined flow resistancebetween the cells of the first and second cores and the outer surface ofthe second layer of porous fibrous woven material which is positionedadjacent the noise to be attenuated is maintained. The attenuationstructure is specifically suitable for use in a severe environment, suchas, high speed gas flow surfaces of modern aircraft. The cells of thefirst and second cores may be of the same or different volumes toattenuate various different noise frequencies.

In manufacturing sound attenuation metal honeycomb sandwich panels whichare exposed to the above mentioned extreme environment while exposed tothe sound produced by modern turbine aircraft engines, it is commonpractice to provide a cellular structure utilizing the Helmholtzresonating cavity principle, wherein a first thin imperforate sheet ofmaterial is bonded to one core surface of a sheet of cellular corematerial and a thin perforate sheet of material is bonded to theopposite core surface.

Panels of this type of construction although satisfactory for a certaindegree of sound attenuation for a narrow range of sound frequencies, arefound to be inefficient noise attenuation structure both for level ofattenuation and for broad band noise frequencies customarily encounteredin and around modern aircraft jet engines. Additionally it has beenfound that the perforations through the perforated sheet exposeddirectly to a high speed flow of air thereacross creates some turbulenceto that air flow and the acoustic performance is generally effected bythe grazing air flow.

Other concepts have been included interposing a sheet of fibrousmaterial between the perforated sheet and the core surface. This hasproven to be unsound acoustically when used adjacent to high speed gasflow.

Attempts to successfully manufacture this and various other adhesivebonded sandwich sound attenuation material of this general type haveresulted in the adhesive used for the bonding to ooze or wick into theperforations and at least partially filling some of these perforationsreducing the effective open area which increases the flow resistancebetween the sound source and the resonating cavities formed by the corecells. When the number and size of the perforations are increased toovercome this deficiency, the structural strength is reduced and airflow turbulence is increased. In those structures where porous fibrousmaterial is utilized within the sandwich between the outer perforatesurface and the central core, the adhesive is found to wick by capillaryaction into the pores and around the fibers of the porous fibrousmaterial as well as the perforations through the perforated materialfurther reducing the sound attenuation effectiveness of the resultingstructure.

SUMMARY OF THE INVENTION

It is the primary object of this invention to produce a double degreehighly efficient sound attenuation structure wherein the perforationsand the porous fibrous material is substantially free of any adhesivematerials.

Another object of this invention is to produce a sound attenuation panelwherein the flow resistance from the outer surface to the stackedhoneycomb core cells can be predicted and controlled.

Still another object of this invention is to provide a double degreesound attenuation material that has structural integrity when utilizedin a severe environment for sound attenuation.

A still further object is to provide an adhesive bonding medium thatprovides isolation between dissimilar metal and provides a funnelingeffect between the perforated sheets and their adjacent sheets of porousfibrous woven material which results in an effectively greater openarea.

These and other objects and advantages of the invention will becomebetter understood by reference to the following detailed descriptionwhen considered together with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first embodiment of the soundattenuation sandwich panel manufactured by the method of the instantinvention.

FIG. 2 is a fragmentary vertical section of the sandwich panel of FIG.1.

FIG. 3 is an enlarged section of FIG. 1 showing the perforation size ofthe central perforate sheet and one of the outer perforated sheets.

FIG. 4 is a perspective view of a second embodiment of the soundattenuation sandwich panel manufactured by the method of the instantinvention.

FIG. 5 is a micrograph of a section of the attenuation panel taken alongline 5--5 of FIG. 4.

FIG. 6 is a flow diagram of the steps of the method of manufacturing theattenuation panels of FIGS. 1 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the various figures in detail, the attenuation sandwichstructure 10 manufactured by the method of the instant inventioncomprises two honeycomb cores 12, 14, each having the usual multiplicityof end wise directed cells 16, 18, a thin imperforate facing sheet 20, apair of perforated facing sheets 22, each having a multiplicity ofperforations 24 of a substantial equal preselected cross sectional area,a single perforated facing sheet 23 having a multiplicity of perforation25 of substantially greater preselected cross-section than those ofsheets 22, and two sheets of porous fibrous material 26, which may bemetallic fibrous felt or any other of a number of various types offibrous material including stainless steel, graphite, nylon or the like.For some specific application, as in the preferred embodiment, a wovenmaterial, such as Dutch twill or the like is preferred with thecrossover contacts of the various strand either bonded together or leftunbonded. The perforated facing sheet 23 provides strength to theassembly and yet, due to the enlarged perforations, does notsignificantly affect the acoustic properties.

The preferred material for the cores and the facing sheets when utilizedin a severe environment, such as an aircraft engine where weight andstrength are critical requirements, is generally aluminum due to itsweight versus strength characteristics. Other metals or materials may beused where the requirements differ. The honeycomb core may beconstructed of phenolic, plastic, paper, Kevlar®, or materials havingthe same or similar properties. Core 12 of the double degree system maybe constructed of aluminum, for instance, the core bonded to theimperforate sheet 20, to provide strength to the ultimate sandwichstructure 10 and the other core 14 could be of an nonstrength providingmaterial as those materials listed above. The cell size (volume) maydiffer between cores 12, 14, or be of equal size depending on the soundfrequency of attenuation interest.

The perforated facing sheets 22 are perforated with a plurality of smallperforations 24, their size, for example, could range from 0.035 to0.065 inches in diameter. The perforated sheet 23 is perforated with aplurality of perforations 25 substantially larger in diameter, theirsize, for example around 0.1875 inches. The perforations 24 provide arange from 10% to 50% actual open area to the perforated sheets 22 whilethe perforations 25 provide approximately 51% open area to perforatedsheet 23 (see FIG. 3 for comparative perforation sizes). Theperforations may be punched, drilled, or chem milled through the sheets.Chem milling is found to be advantageous because the finishedperforation cross sectional area can be predetermined and the facesurfaces do not require deburring, grinding, filing, polishing, etc.,prior to their use in the sandwich structure. The perforations 24 may bespaced, for example, at 0.081 inch intervals, for sheets 22 and 0.250inch intervals for sheet 23 and as an example, perforations 24 may bespaced in rectangular patterns while perforations 25 may be spaced apartin a triangular pattern. Various other spacing intervals or patternsmay, however, be used equally as well to practice this invention. Theimperforate sheet 20 provides a bottom closure for the cells 16 and 18of the adjacent honeycomb cores 12 and 14 which is the lower cavity wallof the combined double degree attenuation panel utilizing the abovementioned Helmholtz cavity resonant principle.

A first thin sheet of stainless steel woven mesh material 26 isadhesively bonded by a layer of adhesive 28 to one surface of each ofthe perforate sheets 22 and the woven mesh surface of one compositesheet 22/26 is likewise bonded to one side of perforated sheet 23. Theadhesive used is typically a nitrile rubber phenolic resin blendcontaining appropriate accelerators and curing agents having thetrademark AF-31 manufactured by the 3M Company, having the trade nameMetal Bond 4021 manufactured by Narmco, having the trademark FM-300manufactured by Bloomingdale Aerospace Products, or any other adhesivehaving the same or similar characteristics as the above mentioned. Theadhesive 28 generally consists of a low solid solvent solution. When thesolvent is removed from these adhesive solutions by evaporation theviscosity index of the remaining adhesive is elevated.

The adhesive 28 for bonding the perforate sheets to the porous wovenmaterials is preferably applied by spraying on the perforated sheets 22,23, surface to be bonded a thin layer of the aforementioned adhesive.The solvent from the adhesive is then removed by evaporation. Thesurface attraction forces cause the adhesive layer around eachperforation to take a smooth rounded, funnel like form when the solventis removed.

The now substantially solid or highly viscous adhesive remaining retainsthat funnel like form or shape, and later during the final cure cycle itsoftens sufficiently to adhere to the porous fibrous woven material.However, the adhesive viscosity is sufficiently high so that it does notwick laterally through the pores of the porous fibrous woven material.

This inherent funneling behavior of the selected adhesive effectivelyenlarges the openings leading into the perforations 24 at the entranceside of the core cells providing an enhanced acoustic open area. Anexample is the use of a perforated sheet having an actual geometric openarea of approximately 34% wherein the final product has an effectiveopen area of approximately 42% or an increase of over 10%. Thiseffective open area can only be attributed to the rounded funnel likeeffect around the opening into the perforated sheet created by the layerof adhesive treated in the manner prescribed. The thickness of thislayer of adhesive is in the range of from 0.0005 to 0.005 of an inch.

This open area effect can be further enhanced by applying a thickerlayer of adhesive 28 in the range of 0.003 to 0.004 of an inch, removingthe solvent, as described above and then curing the adhesive layer. Thethickness of this adhesive layer can be further increased by applyingsuccessive thin layers of adhesive 28 each with subsequent solventremoval and curing. Effective open areas of approximately 50% have beenachieved using a sheet of perforate material 22 having an actual openarea of approximately 34%. In the buildup cure method, discussed above,an additional layer of adhesive 28 is required with the solvent removedbut not cured prior to the panel assembly. The buildup described aboveis unnecessary on the sheet of perforated material 23 because of theinitially large diameter of perforation 25.

The layers of adhesive 28 between the perforate sheets 22, 23 and thesheet 26 of woven material obviously provides a layer of insulationbetween these sheets thus preventing any interaction (galvanic) betweenthe dissimilar metals.

When sizing of the finished panel is required to obtain a desiredspecific configuration by cutting, trimming, etc., or drill throughholes are required, the additional thickness of the adhesive buildupaids in maintaining a continued isolation between the perforate sheetand the porous fibrous woven mesh material.

Another method for providing continuous isolation between the perforatesheet and the porous fibrous woven material where sizing and/or drillingthrough the combined layers is required is the addition of thin layersof nonmetallic cloth material 29, for example, material made fromfiberglass, KEVLAR®, or the like in those areas to be sized or drilledthrough. A thickness in the range of 0.003 to 0.007 of an inch is foundto be satisfactory even when used with single thin layers of adhesive.

METHOD OF MANUFACTURE

Referring now to FIG. 6, the first step of the manufacturing process isto clean the impervious and perforate sheets and the sheets of porousfibrous woven material to ensure that all surfactants have been removedto ensure satisfactory bonding.

The next step of the manufacturing process is to bond the porous fibrouswoven materials to one surface of their respective perforate sheets. Asaforementioned, the adhesive coating for bonding the perforate sheet 14to the porous fibrous material is preferably applied by spraying on thesurface of the perforate sheet a thin layer of one of the aforementionedadhesives 28 at a desired thickness. If multiple coats are desired, thenthe solvent is removed and the adhesive is cured between each successivelayer of adhesive. The final layer of adhesive, as aforementioned, hasthe solvent removed but is left uncured prior to the joining of theperforate sheet to the porous fibrous woven mesh material. When thinlayers of nonmetallic cloth material 29 are required at specificlocations, this material is applied after or during the application ofthe layer or layers of adhesive. The adhesive layer and/or layerssubstantially cover the perforate sheet and coat the strands of thenonmetallic cloth to ensure proper bonding between the perforate sheetand the porous fibrous woven material. The solvent when present in theadhesive is then removed from the last applied adhesive layer ininstances where there are more than one layer as aforementioned. Afterthe solvent of the last applied coat is removed, the porous fibrouswoven material is then placed on the coated surface and a positive forceis applied between the two layers. Pressure may be applied by any wellknown means, such as, but not limited to a press, autoclave or the like.The pressure used is generally in the range of 50 pounds per squareinch.

A primer coat 30 may be applied to the perforate sheet prior to theapplication of the bonding adhesive to improve this bond.

To reduce or substantially eliminate the surface energy and resultingwicking the fibers of the porous fibrous woven material 26 and theadjacent surfaces of the perforated sheet 22 to, which it is attached,are covered with a nonwetting substance, such as Frekote 33, atrademarked product of Frekote, Inc., as well as other materials havingthe same or similar nonwetting characteristics.

After the bond between the perforated sheet 22 and the porous fibrouswoven material 24 is made, the surface of the side of the perforatedsheets with the perforations exposed (the side ultimately to be bondedto a cellular core) is covered with a maskant material which preventsthe nonwetting solution from coating any portion of that surface. It iswell known that any nonwetting (surface energy reducing) materialpresent prevents a satisfactory bond. The maskant material of thepreferred method is a sheet of heavy paper adhered to the perforationexposed surface of the perforated sheet. The adhesive used to attach theheavy paper is nonsoluble when placed in contact with a nonwettingmaterial and is sufficiently tacky to prevent permanent adherence to theperforated sheet. This adhesive however must be sufficiently secured tothe heavy paper so that if completely removed with the paper leaving theperforated surface substantially free of adhesive.

While the maskant material is secured to the perforated sheet, theporous fibrous woven material 26 and the exposed portion of theperforated sheets 22 are saturated with the selected nonwettingsolution, such as Frekote 33, wherein all of the exposed fibers of theporous fibrous woven material and the exposed surfaces, of theperforated sheets are covered. This nonwetting material is then allowedto dry leaving the contacted surfaces covered.

After the nonwetting material is dry, the resistant material is thenremoved by peeling off the heavy paper with the adhesive attachedthereto from the surface of the perforated sheet. Although the adhesivegenerally used is completely removed from the surface of the perforatedsheet any remaining residue should be removed prior to the finalassembly of the components into double layer sandwich attenuation panel.

Referring now to FIGS. 1, 2 and 4 specifically, the various componentsare then stacked in assembly order. A layer of FM150 or an adhesivehaving the same of similar characteristics is applied between theimperforate sheet and the honeycomb core 12 and the perforation exposedsurface of the perforated sheet 22. A layer of the same adhesive is thenapplied between the outer surface of the porous fibrous woven material26 and one surface of honeycomb core 14 or both surfaces of perforatedsheet 23. It should be noted that the nonwetting substance remains onthe fibers of the porous fibrous woven material 26 and the adjacentsurface of the perforated material 22 may include the walls of theperforations of the combined sheet to be bonded to core 14. Although itis well known that the FM150 or the like will not adhere to a surfacecoated with a nonwetting material, it has been found that the cell edgesforce the adhesive to penetrate well into the fibers of the porousfibrous material 26 adjacent thereto. Although the adhesive penetratesbelow and adjacent to the cell edges, it does not wick into the opensurface areas of the porous fibrous woven material nor the perforationsof the perforated sheet because of the coating of nonwetting material.The adhesive selected does not adhere to the fibers, as mentioned above,however, the adhesive does flow around these coated fibers and formmechanical bonds around the fibers and with the layer of AF31 or thelike adhesive. The bonds are structurally sufficiently strong tomaintain physical integrity between the porous fibrous woven material 22and the cell edges of core 14.

A layer of FM150 or the like is then applied between the other surfaceof the honeycomb core 14 and the perforation exposed surface of theother perforated sheet 22.

Pressure in the range of 50 pounds per square inch is then appliedbetween the imperforate sheet 20 and the outer surface of the exposedporous fibrous woven material 26 toward the center of the sandwichstructure. As aforementioned, this pressure may be applied by anyappropriate means as herein discussed.

In some instances it is preferable to cure the adhesive at an elevatedtemperature.

The cured attenuation panel 10 is now ready to be sized as required andplaced into use.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it should be understood that certain changes andmodifications may be practiced within the spirit of the invention aslimited only by the scope of the appended claims.

What is claimed as new and useful and desired to be secured by UnitedStates Letters Patent is:
 1. A method of manufacturing adhesively bondedacoustic attenuation structure, the structural components comprisingfirst and second honeycomb cores with a multiplicity of endwise directedcells, an imperforate facing sheet, a first, second and third perforatedsheets, the third perforated sheet having perforations of a largercross-section than the first and second perforated sheets, a first andsecond thin sheet of porous fibrous material, said method comprising thesteps of:(a) cleaning the components to be assembled into saidacoustical attenuation structure; (b) applying a solvent base firstadhesive covering to one surface of each of said first and secondperforated sheets for bonding thereto one of said first and second thinsheets of porous fibrous material; (c) removing the solvent from saidsolvent base first adhesive; (d) stacking said first perforated sheetwith said first thin sheet of porous fibrous material and said secondperforated sheet with said second thin sheet of porous fibrous material,applying a positive pressure between the components of each stack andcuring said first adhesive; (e) applying a covering of a maskantmaterial to the perforation exposed surface of the now combinedperforate sheets and thin sheets of porous fibrous material; (f)applying an anti-wetting solution to the fibers of the thin sheets ofporous fibrous material and the adjacent attached surface of maskantmaterial covered perforated sheets; (g) removing the covering of maskantof step (e); (h) applying a layer of a second adhesive on one surface ofthe imperforate sheet, the perforated surfaces of the combined first andsecond perforated sheets and porous fibrous material and both surfacesof the third perforated sheet; and (i) stacking the first honeycomb coreadjacent the adhesive applied surface of the imperforate sheet, theperforated sheet and first porous fibrous material adjacent theremaining open surface of the first honeycomb core, the third perforatedsheet adjacent the porous fibrous surface of the combined firstperforated sheet and first porous fibrous material, the second honeycombcore adjacent the third perforated sheet and the perforated surface ofthe com bined second perforated sheet and second porous fibrous materialadjacent the remaining open surface of the second honeycomb core,applying a positive pressure between the outer components toward thecenter and curing the second adhesive.
 2. The method of manufacture ofclaim 1 including an additional step of sizing and forming thecomponents prior to step (a).
 3. The method of manufacture of claim 1,including an additional step of applying an adhesive primer to theperforate sheets prior to applying the first adhesive thereon.
 4. Themethod of manufacture of claim 1 including an additional step ofapplying multiple layers of said first adhesive, removing the solventfrom said first adhesive after step (b) and then repeating step (b)prior to step (c).
 5. The method of manufacture of claim 1 including anadditional step of adhesively bonding a piece of non-metallic cloth atselected locations between the perforate sheets and porous fibrousmaterial after step (b) and before step (c).
 6. The structure resultingfrom the methods of claims 1, 2, 3, 4 or 5.